This invention relates to a beam splitter used when video discs or audio discs are optically reproduced.
FIG. 1 is a schematic perspective view illustrating a player device which operates to reproduce discs of the type identified above. A laser beam emitted from a laser tube 1 is deflected by a beam splitter 2 in the direction perpendicular to the laser beam emitted therefrom. The deflected laser beam is then reflected in mirrors 3, 3' and enters into a light focusing lens 4 by which the laser beam is focused and projected on a disc 5. The thus projected laser beam is reflected on the disc 5 and re-enters into the focusing lens 4. The laser beam as shown by the dashed line reaches the beam splitter 2 via the reflection mirrors 3 and passes straight through the beam splitter 2 onto a photo detector 6.
As evident from the above description, the beam splitter functions both to deflect light entering from the first direction to the second direction perpendicular to the first direction and secondly to pass the light straight entering from the second direction through to the beam splitter 2.
A prior art beam splitter having the above-mentioned functions comprises a pair of prisms 21 and 22, wherein the prisms are combined and adhered by an adhesion material to form a rectangular prism. The rectangular prism thus formed is adhered to a metal chip 7 by the adhesion material to be mounted thereon and the metal chip 7 is fixedly secured to a substrate.
The beam splitter contracts and expands as the ambient temperature varies. For example, due to the heat radiated from the motor of the player device, the ambient temperature of the beam splitter becomes high and thus the rectangular prism tends to expand. Similarly the metal chip expands as the temperature rises. Due to the difference of the heat expansion coefficient between the rectangular prism and the metal chip, the combined surface of the prisms 21 and 22 is peeled off. Accordingly, the beam splitter loses its function.